1. Field of the Invention
This invention relates generally to the field of deflection current generating circuits suitable for use in, for example, a television receiver or a video monitor. More particularly, this invention relates to a television deflection circuit having horizontal pin distortion compensation and reduced power consumption.
2. Background of the Invention
A conventional arrangement of a horizontal deflection circuit and neighboring circuits of a television receiver are shown in FIG. 10. A horizontal deflection circuit 1 is connected with a power supply 3 through a primary coil 2a of a flyback transformer 2. A secondary winding 2b of the transformer 2 is connected to a rectifying/smoothing circuit which is formed of a diode 4 and a capacitor 5. A horizontal deflection current from the horizontal deflection circuit 1 is supplied to a deflection yoke 7 of a CRT 6 having a screen 6a.
FIG. 11 shows the circuit arrangement of the horizontal deflection circuit 1. This horizontal deflection circuit 1 has a parallel circuit of an NPN transistor 22, a diode 23, and a capacitor 24 all in parallel with the series combination of deflection yoke 7 and capacitor 25. The transformer 2 also has a secondary winding 2c, the output of which is rectified by a diode 21.
The operation of this arrangement will be described below. The DC voltage from the power supply 3 is supplied through the primary coil 2a of the transformer 2 to the collector of the NPN transistor 22. This NPN transistor 22 is turned on and off when a signal having a period corresponding to the horizontal deflection period is supplied to its base. The result is that the NPN transistor 22 produces, at its collector, a flyback pulse (retrace pulse) Vcp as shown in FIG. 12(b).
The capacitors 24 and 25 and the deflection yoke 7 make up a resonant circuit, which is resonant at the switching frequency of the NPN transistor 22. Thus, in the deflection yoke 7 a deflection current I.sub.7 of a so-called saw-tooth wave flows. As shown in FIG. 12(a) , this wave linearly increases during the trace interval and linearly decreases during the retrace interval. The electron beam scans (or is deflected) in the horizontal direction on the screen 6a of the CRT 6 under control of this deflection current.
The flyback pulse voltage Vcp is boosted by the secondary coil 2c of the flyback transformer 2, and rectified by the diode 21 so that a high voltage Hv is generated at the cathode of the diode. This high voltage Hv is supplied to the anode of the CRT 6. The voltage induced in the secondary coil 2b is rectified by diode 4 and smoothed by capacitor 5 into a DC voltage Vc. This voltage vc is used for the focus voltage and heater voltage of the CRT 6 and so on.
As shown in FIG. 12(b), the distance from an electron gun 6b to the screen 6a is the shortest at the center of the screen 6a, longer at the top and bottom of the screen 6a, and the longest at the corners of the screen 6a. Consequently, a so-called horizontal pin is generated as illustrated in FIG. 13(a) . If this horizontal pin is left as it is, the displayed image appears distorted. Thus, usually a circuit is added to the horizontal deflection circuit for correcting this horizontal pin distortion.
FIG. 14 shows the principle of a horizontal deflection circuit which is disclosed in Japanese Patent Publication Gazette No. 57-39102. In this example, a DC voltage from a power supply 31 is supplied through a coil 32 to a parallel circuit of a switch 33, the capacitor 24 and a series circuit of the horizontal deflection coil 7 and the capacitor 25. The power supply 31 and the coil 32 correspond to the power supply 3 and the primary winding 2a of the flyback transformer 2 in FIG. 11. The switch 33 corresponds to the parallel circuit of the NPN transistor 22 and the diode 23.
In this embodiment, the parallel circuit (the first parallel circuit) of the switch 33, the capacitor 24 and the series circuit of the horizontal deflection coil 7 and the capacitor 25 is connected to a parallel circuit (the second parallel circuit) of a switch 34, a capacitor 35 and a series circuit of a coil 36 and a capacitor 37. Also, this second parallel circuit is connected in parallel to a series circuit of a coil 38 and a power supply 39. In other words, this horizontal deflection circuit is formed by connecting the first parallel circuit to the second parallel circuit having the same construction as the first parallel circuit.
In order to correct the horizontal pin, it is necessary that the retrace pulse (the voltage V.sub.24 across the capacitor 24), as shown in FIG. 15(a), supplied to the horizontal deflection coil 7, be adjusted in its level to be larger at substantially the central portion (at the central area of the screen) than at the left and right ends (at the upper and lower ends of the screen). The level of a retrace pulse resulting from the addition of this voltage V.sub.24 and the voltage, V.sub.35 across the capacitor 35 is constant as shown in FIG. 15(c) because it is specified by the power supply 31 and the width of the retrace pulse.
Thus, if the voltage V.sub.35 is changed to decrease at the central portion (at the central region of the screen) and increase at the left and right ends (at the upper and lower ends of the screen) in association with the vertical deflection period as shown in FIG. 15(b), the voltage level V.sub.24 can be adjusted to be larger at substantially the central portion (at the central region of the screen) than at the left and right ends (at the upper and lower ends of the screen).
In this example, the switch 34 is operated to switch in synchronism with the switch 33, so that the retrace pulse is generated at the junction between the capacitor 35 and the coil 38. In addition, the voltage from the power supply 39 is changed to decrease at the central portion (at the central region of the screen) and increase at the left and right ends (at the upper and lower ends of the screen) in association with the vertical deflection period as shown in FIG. 15(b). As a result, the level of voltage V.sub.24, as shown in FIG. 15(a), is changed to be larger at substantially the central portion (at the central region of the screen) than at the left and right ends (at the upper and lower ends of the screen) , so that the horizontal pin is corrected.
Other known horizontal pin correction systems are known as the power supply modulating system and the PCT system. The power supply modulating system modulates the voltage of the power supply 3 by the vertical deflection period as, for example, shown in FIG. 11. In the PCT system, as shown in FIG. 11, the transformer has its secondary winding connected in series with the horizontal deflection coil 7, and its primary winding supplied with a vertical deflection current.
In either of these systems, however, the voltage Vcp is modulated by the vertical deflection period, and hence the various voltages taken out of the transformer 2 are modulated by the vertical deflection period. Therefore, these systems can be applied to the so-called separate type circuit arrangement having the horizontal deflection circuit and the high voltage generating circuit separately provided, but cannot be used in the so-called conventional type circuit arrangement having both circuits combined into a single circuit.
The system shown in FIG. 14 can be used not only for the separate type arrangement but also for the conventional type horizontal deflection circuit. In FIG. 14, the various voltages derived from the secondary winding of the transformer, even with the coil 32 connected as its primary winding, are not modulated by the vertical deflection period.
Several significant problems exist with these known circuit arrangements. In the circuit arrangement shown in FIG. 14, it is necessary for current to flow not only in the second parallel circuit of the switch 34, the capacitor 35 and the series circuit of the coil 36 and the capacitor 37, but also in the series circuit of the coil 38 and the power supply 39, and thus the power consumption is relatively large. The present invention reduces horizontal pin distortion at a significantly reduced power consumption.